Paper, board and other fibre-based webs are often surface sized, pigmented or mineral coated to improve characteristics of the paper that affects the printability, such as the surface porosity, the absorptivity, the wettability, or the surface energy (the ink adhesion) of the paper. Today, the printability of uncoated paper is often optimized by the addition of additives to the surface-size or pigmentation recipe. The printability of coated paper is often optimized by optimizing the pigment characteristics, the amount and kind of binders used or by adding additives to the coating compositions. One problem with the addition of additives is that the additives are not always compatible with the other components in the coating, pigmentation or sizing composition.
New printing techniques, such as ink let printing, puts high demands on the printing paper, since the ink must be quickly dried on the substrate and yet provide a high print quality. A desired quality involves a high optical print density, minimized feathering and bleeding and low strike-through. In recent years it has been found that when multivalent salts, such as calcium chloride, are added to the surface size, the applied ink will precipitate fast on the surface of the paper and give rise to a significant improvement in print quality. This is especially advantageous in ink jet printing. U.S. Pat. No. 6,207,258 discloses a composition useful for surface treating a sheet substrate for ink jet printing, the composition comprising a salt of a divalent metal.
Multivalent cations, e.g. calcium, are sometimes added to sizing or coating compositions in the form of lubricants, e.g. calcium stearate. However, the concentration of calcium, in e.g. calcium stearate is not high enough to give rise to the desired effects on the print quality. Thus, the calcium amounts needs to be higher than traditionally used in such products in order to improve the print quality.
One problem with the addition of multivalent salts to coating and/or sizing compositions is that the high concentration of salt needed to achieve the desired effects oftentimes causes rheology problems and undesired precipitations. This is especially a problem when high amounts, such as 0.5-5 parts of salt, is added to anionically charged sizing, pigmentation or coating compositions. Multivalent cations interact strongly with typical anionic-charged polymers or minerals, or additives which are used in papermaking. The stability of anionically charged particles can be improved by e.g. providing steric or electrosteric stability. However, high amounts of electrolytes may cause colloidal flocculation and precipitation.
The printability may further be improved by lowering the pH of a sizing or a coating layer composition e.g. by addition of an acid to the coating or sizing composition. However, not all sizing or coating agents are compatible with low pH. Calcium carbonate pigments can for example not be used at low pH since calcium carbonate dissolves and foam is generated when calcium dioxide is released from calcium carbonate in an acid environment. A reduction in pH may also have a negative impact on rheological properties and on the runnability of the paper machine.
It is an object of the present invention to find a solution to the problem of adding additives, such as salts of multivalent metals, to sizing and/or coating compositions without disturbing the rheological profile of the composition.